American Academy of Health and Fitness
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IN THIS SECTION
Integumentary (Skin) System
Skeletal System
Muscular System
Nervous System
Endocrine System
Cardiovascular System
Lymphatic (Immune) System
Respiratory System
Digestive System
Urinary System
Reproductive System


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Urinary System

The major function of this system is the elimination of excess water, salts, and waste products.

Although renal function declines substantially with age, it usually remains sufficient for removing bodily wastes. Nevertheless, reduced renal function decreases the elderly person's ability to respond to various physiological and pathological stresses. In general, aging is associated with an increased incidence of kidney problems. There is a decline in the number of nephrons by about 30-40% between ages 25 and 85. Nephrons are the basic functional units of the kidneys and are made up of a renal corpuscle and renal tubules. There is also a reduction in filtration rate due to cumulative damage to the system and to reduction in blood flow to the kidneys.

Urine is formed by the kidneys through three processes: filtration, reabsorption and secretion. The ultimate source of urine is blood, and a large amount of blood is transported to the kidneys for filtration by way of the renal arteries. Renal blood flow progressively decreases from 1200 mL/minute at age 30 to 40 years to 600 mL/minute at age 80. A decrease in the glomerular filtration rate, or the rate at which blood is forced through and filtered by the renal corpuscle in the kidneys, is the most important functional defect caused by aging.

 

Fun fact:

A typical adult kidney measures approximately 4 inches in length and 1.2 inches in thickness. This is about the size of a large bar of bath soap.


Not so fun fact:

Elderly people on salt-restricted diets have a decreased ability to conserve sodium. This can contribute to the increased tendency of elderly persons to develop dehydration. Still, the most important cause of dehydration is the loss of the sensation of thirst, a characteristic common in the elderly. Loss of thirst is especially important when elderly persons are confronted with an illness or an infection that increases demands for or limits the intake of salt and water.

Since the kidneys receive up to 25% of the resting cardiac output through the renal arteries, a large portion of total blood volume is filtered through the renal corpuscle each day. At this rate, it wouldn’t take long to totally deplete the body’s entire blood volume. Since this is obviously not a common problem, the body must have some way to recover a majority of that filtered blood. The process that returns necessary items from the filtrate back into the blood is called reabsorption.

Reabsorption is responsible for returning 99% of filtrate back to the circulatory system. Reduced nephron sensitivity which occurs with age, however, results in less reabsorption of water, so urination becomes more frequent while daily fluid requirements increase. At the same time, there is usually a loss of thirst which compounds the problem. The muscles that help regulate the release of urine become weaker which leads to problems with incontinence and often involves slow leakage of urine. This is usually a more common problem in women. In males, urinary retention might develop due to chronic inflammation of the prostate gland. Swelling of the gland prevents the flow of urine as it puts pressure on the urethra.

 

 
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